1. Field of the Invention
The invention relates to a fluidized bed reactor and a method for producing granular polysilicon.
2. Description of the Related Art
Polycrystalline silicon granules, or polysilicon granules for short, are an alternative to polysilicon produced in the Siemens process. Whereas the polysilicon in the Siemens process is obtained as a cylindrical silicon rod which, before further processing thereof, must be comminuted in a time-consuming and costly manner to what is termed chip poly, and possibly must again be purified, polysilicon granules have bulk material properties and can be used directly as a raw material, e.g., for single crystal production for the photovoltaics and electronics industries.
Polysilicon granules are produced in a fluidized-bed reactor. This is achieved by fluidizing silicon particles by means of a gas flow in a fluidized bed, where the fluidized bed is heated to high temperatures by a heating device. By addition of a silicon-containing reaction gas, a pyrrolysis reaction proceeds at the hot particle surface. In this process, elemental silicon is deposited on the silicon particles and the individual particles grow in diameter. Via regular take off of particles that have grown and addition of smaller silicon particles as seed particles (in the further course of this document termed “seed”), the method can be operated continuously with all of the associated advantages. As silicon-containing reactant gas, silicon-halogen compounds (e.g. chlorosilanes or bromosilanes), monosilane (SiH4), and also mixtures of these gases with hydrogen are described. Such deposition methods and devices for this purpose are known, for example, from U.S. Pat. No. 4,786,477 A.
US 2008299291 A1 describes a fluidized-bed reactor and a method for producing polysilicon granules. The reactor is divided into two zones. The first zone is weakly fluidized using a silicon-free bottom gas. The reaction gas is injected into the following reaction zone. This arrangement is known as a bubble-forming fluidized bed having additional vertical secondary gas injection. Above the reaction gas nozzles, local reaction gas jets form, within which the silicon-containing gas is deposited on seed particles at temperatures between 890 and 1400° C.
It is disclosed that establishing the bottom gas velocity in a ratio to the reaction gas velocity is of critical importance in order to maximize the product quality and the yield of the target product polysilicon granules.
In addition to the velocity ratio, the mass streams of the reactant gases, the temperature, the particle size and the height of the reaction zone must also be established optimally.
Also bubble formation in this context is a critical parameter. In addition, for optimum development of the gas jets, a well-defined geometric arrangement of the nozzle spacings from one another and the spacings of the nozzles from the wall must be selected. The spacing between the nozzles should be selected in such a manner that the ratio of nozzle spacing to nozzle diameter (internal diameter of the nozzle at the site of gas exit into the fluidized bed) is greater than 7.5.
Undesirable byproducts are silicon dust from homogeneous gas-phase deposition, and silicon dust from abrasion and wall deposition.
It is known that the product quality established in the polysilicon product, in particular, also the chlorine content, depend on the process conditions in the fluidized-bed reactor.
U.S. Pat. No. 6,007,869 claims polysilicon granules having a chlorine content of 6-47 ppmw. Owing to the low chlorine content, adverse effects during CZ-pulling, such as poor monocrystal quality, spray effects and formation of corrosive gases, are avoided. For producing the polysilicon granules, a reaction gas temperature of greater than 900° C. and a particle temperature of greater than 1000° C. must be selected.
U.S. Pat. No. 5,037,503 discloses the production of silicon monocrystals using polysilicon granules having a chlorine content of less than 15 ppmw (deposition with trichlorosilane) or a water content of 7.5 ppmw (deposition with silane). When such polysilicon granules are used, no spray effects occur during crystal pulling.
US 2012230903 A1 discloses a fluidized-bed reactor having a gas distributor for gas distribution into the reaction chamber of the reactor, comprising a multiplicity of distributor openings which ensure a fluid-communicating connection between a first gas source, a second gas source and the reaction chamber, wherein the distributor openings each have at least one central opening and one decentralized opening, wherein the decentralized openings are fluid-communicatingly connected only to the first gas source, but not to the second gas source. Deposition of silicon on the reactor wall is said to be avoidable by the reactor described.
In U.S. Pat. No. 7,927,984 B2, a fluidized-bed reactor that is conical in the lower part is disclosed which is distinguished in that the distributor plate in plan view is divided into central nozzle openings for feeding reaction gas. In addition, around the central nozzles, further nozzles are arranged which serve for metering etching gas into the reactor. It is described that there can be a plurality of reaction gas nozzles and that the reaction gas nozzles need not necessarily be arranged in the center.
US 2012269686 A1 discloses a fluidized-bed reactor having a base plate containing gas channels, wherein a multiplicity of gas nozzles are uniformly distributed on the base plate and the base plate is subdivided into a plurality of regions, wherein in each region, in each case the same number of gas nozzles is present. The number of regions can correspond to the number of gas channels.
It is known from the prior art that various parameters must be optimized for a polysilicon granule production method that is improved with respect to product quality. It is desirable to achieve optimum reaction conditions in the reaction zone of the fluidized-bed reactor and to maintain them in a stable manner during the deposition process.
It is known in this context from the prior art that geometric arrangements of the reaction nozzles and fluidizing nozzles play a role for optimizing the gas jets and also the streams of the media and energies supplied to the reactor.
A high reaction gas concentration, a high solids concentration, and a high temperature in the reaction zone should be present simultaneously.
For further optimization it would actually be necessary to know the parameters in the reaction zone and their superimpositions by in-situ measurements. Unfortunately, conventional fluidized-bed reactors for deposition of polysilicon granules do not allow for the possibility of determining the corresponding parameters metrologically. This is due to the fact that the required measurement instruments would not withstand the reaction gas mixture. Secondly, the product quality of the granule particles coming into contact with the internals would be impaired.
The objective of the invention resulted from the problems described.